Process for the production of ethanol

ABSTRACT

In the production of ethanol by continuous fermentation in a fermentor (4) with continuous stillage recirculation (19, 20) to the fermentor, the fermentation liquid (6) continuously withdrawn from the fermentor is first sieved in a straining step (7) for separation of coarse solid particles. Then the fermentation liquid (11) is separated in a yeast separation step (12), from which a yeast stream (13) is recirculated to the fermentor (4) and a yeast-free stream (14) is fed to a primary distillation step (9). From the bottom stream (16) from the distillation step (9) a part (19, 20) is recirculated to the fermentor and another part is subjected to final stripping in a secondary distillation step (10). By installing a further centrifugal separation step (22) in the stream (14), which is fed to the distillation step (9), or in the bottom stream (16) which leaves the distillation step (9), finer inert solid particles can be removed from the circulation circuit comprising the distillation step ( 9) and the fermentor (4). Thereby a considerable saving of separator capacity in the yeast separation step (12) and an improved fluidity in the system can be achieved.

The present invention relates to a process for the production of ethanolby continuous fermentation of a carbon hydrate containing substrate in afermentor, in which process a stream of fermentation liquor iscontinuously withdrawn from the fermentor and divided in a centrifugalseparation step into a yeast enriched stream, which is recirculated tothe fermentor, and into an essentially yeast-free stream, which isdivided in a primary distillation step into a top stream enriched inethanol and a remaining liquid bottom stream, of which a part isrecirculated to the fermentor and the remaining part is fed to asecondary distillation step for division into a vapour stream containingthe remaining ethanol and an ethanol impoverished stillage stream.

In a known continuous ethanol fermentation process of the kindintroductively mentioned, such as disclosed in applicant's internationalapplication WO No. 83/01627, coarse solid particles are first separatedin a straining step from the fermentation liquor continuously withdrawnfrom the fermentor, whereafter the stream that passes through the sievesis fed to a centrifugal separation step for separation into a yeastphase, which is recirculated to the fermentor, and a phase freed fromyeast, which is fed to a primary distillation step in a distillationplant. Part of the solid, non-fermentable or inert material continuouslyfed to the process with the raw material is discharged from the processcircuit in the form of coarse particules, which as sieve rejects isdischarged from the circulation loop, which comprises the fermentor andthe primary distillation step. The remaining part of inert solidmaterial that continuously must be removed, is discharged with that partof the bottom stream from the primary distillation step which is fed toa secondary distillation step, also called stripping step, in which theremaining ethanol is stripped off and the liquid stream is concentratedto a final stillage stream. As a consequence thereof, the part of thesolid inert material which cannot be screened off with the sieves must,at steady-state condition, reach a certain concentration in the bottomstream from the primary distillation step to be discharged in thecorrect amount from the process circuit through the stillage stream fromthe stripper.

In certain cases, such as in fermentation on grain raw material, aconsiderable concentration of fine particles arises in the streams toand from the primary distillation step, for example in the range of 4-6%by weight. Moreover, it can be foreseen that a still higherconcentration of these "fines" must be built up in the fermentor-yeastseparator-circuit, since the inert particles further tend to be enrichedin the yeast phase, which is recirculated from the periphery of theyeast separators to the fermentor. Since further the proportion in thefermentation liquor from the fermentor normally amounts to no more than2% by weight DS (dry solids), the sludge capacity of the yeastseparators will to a large part be occupied by the inert material. As aconsequence, the very large part of the flow fed to the yeast separator,at grain fermentation in the range of 60-70% , must be recirculated assludge phase to the fermentor. Therefore large separator capacity isacquired, since the amount of sludge in this case is the dimensioningfactor.

One way to reduce the need for yeast separator capacity is to change theflow rate between effluent and sludge phase, that is to increase theeffluent flow and decrease the sludge, flow. Thereby the requiredconcentration of inert solid material in the stream to and from theprimary distillation step could be achieved simultaneously as a lowersteady-state concentration of solid inert material in thefermentor-yeast separator circuit would be required, which could makepossible a reduction in yeast separator capacity. However, thedisadvantage of such a modification is increased yeast losses, whicharise due to increased yeast drainage with the effluent to the primarydistillation step, in which alive yeast is killed off.

The object of the present invention is to reduce the concentration ofsolid inert material in the fermentor-yeast separator circuit and toreduce the required yeast separator capacity while maintaining the yeastlosses at an unchanged low level.

This object is reached according to the invention in a process of thekind introductively mentioned by dividing in a further centrifugalseparation step at least a part of the liquid stream fed to the primarydistillation step or the liquid stream discharge from the primarydistillation step into a stream impoverished in fine particles and asludge stream enriched in fine particles and discharging said sludgestream from the circulation circuit, which comprises the primarydistillation step and the fermentor.

According to a preferred embodiment of the invention, the furthercentrifugal separation step is located after the primary distillationstep. Thereby, at least two advantages are reached compared with thecase of locating the centrifugal separator before the primarydistillation step. One advantage resides in the fact that the separatedsludge concentrate is comparatively impoverished in ethanol and can befed directly to the secondary distillation step for final strippingtogether with the sieve rejects and possible remaining liquid streamfrom the primary distillation step. A second advantage is that some partnonseparable protein in solved or colloidal state in the feed streem tothe primary distillation step is transferred into separable form throughcoagulation due to heating during heat exchange with the recirculationstream from the primary distillation step and further during heating inthe distillation step itself. These protein aggregates thus formed cannow be PG,5 separated off directly and do not have to be recirculated tothe fermentor.

In the case a further centrifugal separation step is located before theprimary distillation step, it is obtained in the separated sludge phasean ethanol concentration of about the same magnitude as that of flow fedto the primary distillation step, that is normally in the range of 4-6%by weight. If the separation conditions are selected so that theconcentration of solid material in the sludge phase from the separationstep is high and the total sludge stream therefore can be keptrelatively small, a sludge phase can be fed directly to the strippingstep despite its high ethanol concentration without signaificantlyimpairing the ethanol yield. If the sludge phase is comparatively large,that is if it comprises a significant part of the liquid to be strippedto final stillage, a feasible way to avoid impaired ethanol yield is tostrip off most of the ethanol in the sludge phase in a separate smallercolumn. The bottom flow from this column can then suitably be fed to astripping step also used for stripping sieve rejects and the possiblefurther part of the bottom stream from the primary distillation step.

The further centrifugal separation step according to the invention makespossible a considerable reduction of solid DS in the circulation circuitcomprising fermentor, yeast separator and primary distillation step. Themost conspicuous effect thereof is that the flow ratio between effluentand sludge phase from the yeast losses, which, at unchanged ethanolproduction, makes possible a considerable reduction of the sludge flowrecirculated to the fermentor as well as of the feed flow to the yeastseparators. If for example the amount of inert DS to the yeastseparators is reduced from e.g. 8% by weight to 3% by weight byinstalling a centrifugal separator according to the invention, therequired yeast separator capacity can be reduced to about half, whichmeans a considerably reduced investment and energy costs, since therequired further separator capacity is far lower than saved yeastseparator capacity.

The possibility of maintaining a lower concentration of solid inert DSin the circulation circuit due to the invention, provides severalfurther improved process conditions. The fermentation environment in thefermentor is improved, i.e. due to less foaming and facilitatedstirring. Less contamination is obtained in the primary mash column andalso in further process units such as heat exchangers for heat exchangebetween the cold yeast-free stream from the yeast separators andrecirculation stream from the primary mash column. A lower viscosity ofthe streams in the circulation circuit improves the fluidity, whichfacilitates the straining operations and pumping.

The invention will now be further illustrated by means of a fewembodiments of the same, shown as examples, reference being made to theaccompanying drawing, in which

FIG. 1 shows a flowsheet with an extra separator installed after theprimary distillation step, and

FIG. 2 shows a flowsheet with an extra separator installed before theprimary distillation step.

According to the flowsheet of FIG. 1, fermentation raw material, e.g.milled grain, and required process water are supplied with a stream 1 toa substrate treating step 2, in which enzymatic hydrolysis tofermentable sugars occurs. From the substrate treating step 2 a stream 3with hydrolysate is fed to a fermentor 4, in which continuousfermentation of the hydrolysate occurs at steady-state conditions bymeans of yeast suspended in the fermentation liquid during formation ofethanol and carbon dioxide, which is discharged from the top of thefermentor through 5. To maintain constant yeast concentration in thefermentor, air or oxygen is supplied either to the fermentor feed flow 3or to the fermentor itself to achieve a yeast growth corresponding tominor yeast losses. A stream of fermentation liquor 6 containing ethanolof a concentration in the range of 4-6% by weight is continuouslywithdrawn from fermentor 4. The stream 6 is fed to a straining step 7for separating off a sieve reject stream 8, which is withdrawn from thecirculation circuit comprising fermentor 4 and a primary distillation 9and fed to a secondary distillation step 10.

A stream of fermentation liquor 11 freed from coarse particles andfibres is fed to a yeast separation step 12 comprising one or severalyeast separators. A heavy phase stream 13 containing essentially allyeast from the stream 11 and also finer inert material not rejected inthe straining step 7 is recirculated to fermentor 4. A light phasestream 14 essentially free from yeast is continuously withdrawn fromyeast separators 12 and fed to the primary distillation step 9,generally consisting of a multi-stage column. From the top of column 9the major part of the ethanol present in the yeast-free stream 14 isremoved through a vapour stream 15, which normally contains ethanol inthe range of 35-40% by weight. A bottom stream 16 having an ethanolconcentration in the range of 0.1-0.2% by weight is discharged from thebottom of column 9. In the shown embodiment a partial stream 17 of thebottom stream 16 is sent directly to a stripping column, constitutingthe secondary distillation step 10. Another partial stream 18 of thebottom stream 16 is recirculated to the fermentor through 19 and/or alsoto the substrate treating step 2 through 20. A further partial stream 21of the bottom stream 16 is fed to a further centrifugal separation step22, in which it is divided into a sludge stream 23 enriched in fineparticles and an effluent stream 24. The sludge stream 23 is fed to astripping column 10, and the effluent stream 24 is recirculated with thestream 18 in the circulation circuit comprising fermentor 4 and theprimary distillation step 9.

For heat exchanging the yeast-free stream 14 to be fed to the primarydistillation step 9 with the part of the bottom stream 16, which isrecirculated to fermentor 4 and/or the substrate treating step 2, a heatexchanger 25 can be installed for heat exchanging the recirculationstreams 18 and 24 with the yeast-free stream 14, whereby the inflow tothe sludge separator 22 will be warm. Alternatively the heat exchangecan be carried out in a heat exchanger 26 installed before the sludgeseparator 22, whereby the inflow to the same will be cooled down to nearfermentor temperature. A warm inflow 21 to the separator 22 canfacilitate the separtion, while high temperature operation puts higherdemand on the separator functioning from the view of safe operation.Whether heat exchange is to be carried out before or after the separator22 depends on the type of separator used, and in many cases a divisionof the heat exchange through one unit 25 and one unit 26 is to bepreferred.

If the separation conditions in the centrifugal separation step 22 isselected so that the sludge stream 23 has relatively low sludgeconcentration, the stream 17 to the stripper 10 can possibly beeliminated. On the contrary, if the sludge stream 23 has a highconcentration of inert solid material, the sludge stream 17 will benecessary for balancing the removal of inert material from thecirculation circuit and maintaining steady-state.

Likewise depending on the separation conditions in the centrifugalseparation step 22, whole or part of the stream recirculated to thefermentor or to the substrate treating step can be passed through theseparator 22. In the extreme case the stream 18 can thus be eliminated.Further, an arbitrary part of the recirculation streams 18 and 24 can beused for washing (not shown in FIG. 1) the sieve reject stream 8 fromstraining step 7 in order to reduce the yeast losses and the ethanolconcentration in the sieve reject stream 8.

The sieve reject stream 8 as well as the sludge stream 23 from thecentrifugal separation step 22 and the stream 17 are fed to thestripping column 10 for stripping off the remaining ethanol andrecovering a concentrated stillage stream 27 from the bottom of thestripping column 10. The ethanol containing vapours 28 from thestripping column 10 are fed to the primary distillation column 9 asdirect heating medium. The stripping column 10 and the primarydistillation column 9 can suitably be combined in a common column, inwhich the downwards streaming liquid flow is blocked on an intermediatelevel for the discharge of the bottom stream 16 from the upper part 9 ofthe column, which constitutes the primary distillation step.

FIG. 2 shows an embodiment, which is identical with the embodiment shownin FIG. 1 except the location of the sludge separator 22 in the processcurcuit. Therefore, the exactly corresponding process units and streamshave been given the same figure references as in FIG. 1.

Depending on the selection of sludge separator and separationconditions, whole or part of the yeast free stream 14 from the yeastseparation step 12 is fed as feed flow 21a to a further sludge separator22a. A sludge stream 23a enriched in solid inert material is dischargedfrom the separator 22a and fed to stripping column 10. Since now thesludge stream 23a, contrary to the previous embodiment, has aconsiderable ethanol concentration being of the same magnitude as thatof the inflow 14 to the primary column 9, it can be convenient to add tothe stripping column 10 one or a few further distillation trays incomparison with the stripping column of FIG. 1, and to supply the sludgestream 23a on a somewhat higher level to the stripping column 10 thanthe remaining streams 8 and 17 also fed to the stripping column 10. Astream 24a impoverished in fine particles is sent together with theremaining part 18a of the yeast free-stream 14 to the primary column 9.

Of the same reason as mentioned in context with the previous embodiment,one or both of the by-pass streams 17 and 18a can possibly be eliminateddepending on the selection of separation conditions and centrifugalseparator.

I claim:
 1. In a process for the production of ethanol throughcontinuous fermentation of a carbohydrate containing substrate in thepresence of yeast in a fermentor, wherein a stream of fermentationliquor is continuously withdrawn from the fermentor, said stream offermentation liquor is divided in a first centrifugal separation stepinto a yeast enriched stream, which is recirculated to the fermentor,and an essentially yeast free stream, said yeast free stream is dividedin a primary distillation stage into an overhead stream enriched inethanol and a liquid bottoms stream at least a part of which is recycledto the fermentor, the improvement which comprises straining the streamof fermentation liquor prior to said first centrifugal separation stepto separate a stream of coarse particles, centrifuging the yeast freestream prior to said primary distillation stage or centrifuging theliquid bottoms stream directly after removal from the primarydistillation stage, to give a stream impoverished in fine particles anda sludge stream enriched in fine particles, blending said stream ofcoarse particles with the stream enriched in fine particles to form acombined stream, subjecting said combined stream to distillation in asecondary distillation stage, returning a vapor stream from saidsecondary distillation stage to said primary distillation stage,removing a sludge stream from said secondary distillation stage anddischarging the sludge stream from the secondary distillation stage fromthe circuit which includes the primary distillation stage and thefermentor.
 2. In a process for the production of ethanol throughcontinuous fermentation of a carbohydrate containing substrate in thepresence of yeast in a fermentor, wherein a stream of fermentationliquor is continuously withdrawn from the fermentor, said stream offermentation liquor is divided in a first centrifugal separation stepinto a yeast enriched stream, which is recirculated to the fermentor,and an essentially yeast free stream, said yeast free stream is dividedin a primary distillation stage into an overhead stream enriched inethanol and a liquid bottoms stream at least a part of which is recycledto the fermentor, the improvement which comprises centifuging the liquidbottoms stream directly after removal from the primary distillationstage to give a stream impoverished in fine particles and a sludgestream enriched in fine particles, feeding said sludge stream to asecond distillation stage, stripping ethanol and water from said sludgestream in said second distillation stage, to form an ethanol containingstream, charging the ethanol containing stream from said seconddistillation stage to said first distillation stage and discharging saidstripped sludge stream from the circuit which includes the primarydistillation stage and the fermentor.